Who discovered dark matter in galaxies?

Many astronomers get their history of the subject from text books, and generally discoveries are presented as neat, tied up bags with statements like "X discovered Y". However, now and again, people look more deeply and the story is always more complex.

I decided to look at the question of who discovered dark matter in galaxies as the textbook give a very "X discovered Y" answer to this, and gave a brief presentation at our Astro Morning Tea. But I am not a historian, and this is not complete, but just what I found after a couple of hours of looking around (thanks Brad for his input).

OK, let's start with dark matter. The first clues about the existence of dark matter was not in galaxies, but in clusters of galaxies (and this statement ain't that simple). The person generally credit is given to this man, Fritz Zwicky,

He was looking at the motions of galaxies in clusters and concluded that there is more mass present that indicated by visible light, and that "dark matter" was needed to hold the clusters together.

The textbook answer for galaxies beings in 1970 with the work of this famous astronomer, Vera Rubin,

with her collaborator, Kent Ford, she measure the speeds that stars were moving in our nearest cosmic companion, Andromeda galaxy, both in the centre, and then out into the disk. What did they find? Well, heres the rotation curve;

As you can see, stars are moving around at a few hundred kilometres per second. Importantly, however, as we go to larger distances, the speed doesn't drop. In fact, it stays roughly constant. If the only mass in Andromeda was associated with the stars, then it should drop off, so there must be more matter there than we can see.

With a decade's more work, Rubin had shown that Andromeda was not a one-off, but these constant rotation curves were seen in all of the spiral galaxies they looked at

and when we had radio observations even beyond the stars, we could also see that the gas was moving around that the same speed as the stars, at much larger distances.

(Ken is the one in the middle :) The prize was for Ken's work in 1970, where (as the snippet from his paper shows) he discovered evidence for dark matter is galaxies, based upon how the light is distributed. Remember, this was the same time that Rubin and Ford were doing their work on Andromeda!

But reading Ken's paper reveals that he cites earlier work that suggested that there is more mass in galaxies than is visible, one of them from my own The University of Sydney. Here's the paper, from 1966.

The important number in there in the summary is the MT/L which is the total Mass to Light Ratio; if this was about unity, it would mean there is as much mass there as indicated by the light, but we see that in this galaxy, the lovely NGC 300, this ratio is almost 11!

The even more interesting thing, is that this paper cites an even earlier work, from 1961

who calculated that this value is 9, again indicating that there is more mass than is apparent from the distribution of the light.

At this point, I had to get back to work, and I didn't chase it further, but it should be clear that the answer to "who discovered dark matter in galaxies?" is not a clear cut question, with a straight-forward answer. This is not to take away from the great work done by Rubin and her collaborators (which is often the textbook and wikipedia answers), but just to illustrate that the actual history of scientific discovery is often messy.

If you are interested (and you should be) I recommend you have a read of these superb lecture notes by my colleague, Joss Bland-Hawthorn, especially the appendix that looks at this question in a lot more detail. It doesn't make things simpler :)

Well, that's slightly messy two, as Oort the year before suggested more matter was needed in the galaxy (by a factor of a couple) to explain star motions, but I think Zwicky is the correct person to be credited with the discovery of lots of dark matter (although, the historians of science should feel free to prove me wrong :)

A bit further down in the Google Books version of Life magazine pointed to by your link, there is an article mentioning and picture of Joan Baez (who, when both were young, had a slight similarity in appearance to Virginia Trimble). Joan Baez's cousin, noted proto-blogger John Baez, is, like Virginia Trimble, a professor at the University of California.

And what a blast from the past that Life magazine is. Those who think there are too many advertisements on the web should get a load of this! (Of course, in a magazine I can just flip the page, while many web ads are very annoying.)

correct me if I am wrong ... the snippet shown from Freemans paper was only a footnote, not the main topic of the paper at all ... the majority of citations to that paper are for "Freemans Law" long since shown not to hold true for all cases.

Perhaps history will show that there is no such thing as dark matter, thus solving any confusion.

Post a Comment

Popular posts from this blog

Proton: a life story by Geraint F. Lewis1035 years: I’ve lived a long and eventful life, but I
know that death is almost upon me. Around me, my kind are slowly melting into
the darkness that is now the universe, and my time will eventually come. I’ve lived a long and
eventful life…

10-43 seconds: A time of unbelievable light, unbelievable
heat! I don’t remember the time before I was born, but I was there,
disembodied, ethereal, part of the swirling, roaring fires of the universe coming
in to being. But the universe cooled. From the featureless
inferno, its character crystalized into a seething sea of particles and forces.
Electrons and quarks tore about, smashing and crashing into photons and
neutrinos. The universe continued to cool. 1 second: The intensity of the heat steadily died away, and I was born. In
truth, there was no precise moment of my birth, but as the universe cooled my
innards, free quarks, bound together, and I was suddenly there! A proton! But my existence seemed fleet…

Everyone loves black holes. Immense gravity, a one-way space-time membrane, the possibility of links to other universes. All lovely stuff.

A little trawl of the internets reveals an awful lot of web pages discussing black holes, and discussions about spaghettification, firewalls, lost information, and many other things. Actually, a lot of the stuff out there on the web is nonsense, hand-waving, partly informed guesswork. And one of the questions that gets asked is "What would you see looking out into the universe?"

Some (incorrectly) say that you would never cross the event horizon, a significant mis-understanding of the coordinates of relativity. Other (incorrectly) conclude from this that you actually see the entire future history of the universe play out in front of your eyes.

What we have to remember, of course, is that relativity is a mathematical theory, and instead of hand waving, we can use mathematics to work out what we will see. And that's what I did.

First, the usual apologies! It's been an age since I have written here, but, as you know, the life of the academic is a busy one! Especially since I have just completed a book which is to be published next year. More on that journey later, but today a little post about academic toolkits.

This is something that I have written about before, and I know some of my colleagues and peers disagree with me, but that's fine as I think it illustrates that there is no single recipe for success in academia (Am I a success in academia? That's for others to judge, but I am still here after twenty years :).

What makes a "good" academic? In modern academia, we have to be specialists, focused on a generally tiny part of the immense enterprise called science. When ever I realise this, Kenneth Williams springs immediately to mind
Crossing boundaries and commenting on other areas of science that are not in your domain is met with suspicion and attack, and it's not just new ideas…